1. Field of the Invention
Embodiments of the invention described herein pertain to the field of antennas. More particularly, but not by way of limitation, one or more embodiments of the invention enable a multi-feed dipole antenna and method covering a wide frequency band.
2. Description of the Related Art
One of the simplest antennas is a dipole antenna. The length of a typical dipole antenna generally approximates one half-wavelength of a desired transmit/receive frequency. The radiation pattern provided by a dipole antenna is limited when the frequency of the signal at the antenna is high enough to assert harmonic modes in the current distribution. The pattern is said to “bifurcate” at the higher frequencies and results in a pattern that causes side lobes and nulls to appear where they would not appear at lower frequencies.
Dipole antennas are limited in terms of their radiation pattern and impedance bandwidth. Radiation pattern bandwidth is that range of frequencies over which the radiation pattern is substantially constant and is within specifications. The impedance bandwidth is that range of frequencies over which the input impedance is with a certain acceptable ratio of the nominal impedance. This is often described in terms of standing-wave ratio (SWR), where an acceptable maximum SWR may be 2.5:1 or some other value. Generally, radiation pattern bandwidth and impedance bandwidth are the primary considerations for wide band antennas. An antenna with a wider bandwidth than a thin wire dipole antenna is a “fat dipole”. Antennas designed for volumetrically compact wide band coverage generally follow the fat dipole approach since biconical wide band antennas take up such a large volume. Once the wire conductors of a dipole antenna are made thicker, the antenna bandwidth increases.
U.S. Pat. No. 3,000,008 to Pickles describes a variant of a fat dipole antenna. Pickles '008 is directed at a narrow-band decoupling mechanism that attempts to maximize current flow on the antenna and not on the support structure of the antenna. The antenna has a half wavelength tube coupled directly to two other shorter tubes that act as chokes, preventing current flow onto the support structure. The feed for the antenna is symmetric. Excitation voltages are directly “applied across the gaps” between the different tube sections. The main problems with this antenna are the presumption of symmetry in the support structure on either side of the antenna. Also, the two feed points (gaps) are at a very high impedance and will present practical difficulties in impedance matching. Further, the Pickles antenna is based upon very narrow-band structures such as quarter wave chokes, and is therefore not a wideband design.
Johnson ISBN 0-07-032381-X in “Antenna Engineering Handbook” provides a thorough background of dipole antennas including cylindrical dipoles, biconical dipoles, folded dipoles and sleeve dipoles. Kraus, et al., ISBN 0-07-232103-2 in “Antennas for all Applications” published by McGraw-Hill Companies, Inc., shows various sleeve dipoles antennas. The antennas described in these references fail to achieve maximum volumetric efficiency. Johnson shows an open-sleeve dipole which does not make efficient use of a cylindrical volume, as the dipole elements are thin after exiting the sleeve. Kraus shows a quarter wave sleeve monopole where the sleeve and the upper radiator are the same diameter, but the interior is not efficiently used for impedance matching. Further, the quarter wave sleeve monopole requires a large ground plane for operation. The present invention is ground plane independent.
U.S. Pat. No. 4,087,823 to Faigen et al., describes another variant of a fat dipole. Specifically, Faigen et al., '823 describes a device that is approximately three quarters of a wavelength long with a central section filled with dielectric. The antenna is driven asymmetrically with lines directly extending across the gaps to drive the various tubes. One problem with this antenna is the use of an asymmetrical feed structure. This may allow the pattern to vary over frequency, limiting the pattern bandwidth. Another problem is the use of heavy and expensive dielectric material to load the inside of the center tube so that it internally operates as a half-wavelength section. For at least the limitations described above there is a need for a multi-feed dipole antenna and method.